Reference-frequency-insensitive phase locked loop

ABSTRACT

A phase locked loop may be operable to generate, utilizing a frequency doubler, a reference clock signal whose frequency is twice a frequency of a crystal clock signal and is keyed on both rising and falling edges of the crystal clock signal. A sampled loop filter (SLPF) in the phase locked loop may capture charge from a charge pump (CHP) in the phase locked loop and the charge is captured at a frequency corresponding to the frequency of the reference clock signal. Opening a switch of the SLPF may hold the captured charge during a phase comparison and closing the switch may release the captured charge. The switch is controlled utilizing a control signal. By utilizing the SLPF in the phase locked loop, the phase locked loop may eliminate, at an output of the CHP, disturbance which is associated with duty cycle errors of the crystal clock signal.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application is a continuation of U.S. application Ser. No.14/860,262 filed Sep. 21, 2015, which is a Continuation of U.S.application Ser. No. 14/452,204 filed Aug. 5, 2014, which in turn makesreference to, claims priority to and claims the benefit of U.S.Provisional Patent Application Ser. No. 61/867,333 filed on Aug. 19,2013.

Each of the above stated applications is hereby incorporated herein byreference in its entirety.

FIELD OF THE DISCLOSURE

Certain embodiments of the disclosure relate to communications and/orsignal processing. More specifically, certain embodiments of thedisclosure relate to a method and system for areference-frequency-insensitive phase locked loop.

BACKGROUND OF THE DISCLOSURE

Existing methods and systems for conventional phase locked loops (PLLs)can be costly, cumbersome and inefficient, and conventional phase lockedloops (PLLs) may be sensitive to duty cycle errors associated with areference clock. Further limitations and disadvantages of conventionaland traditional approaches will become apparent to one of skill in theart, through comparison of such systems with the present disclosure asset forth in the remainder of the present application with reference tothe drawings.

BRIEF SUMMARY OF THE DISCLOSURE

A system and/or method for a reference-frequency-insensitive phaselocked loop, substantially as shown in and/or described in connectionwith at least one of the figures, as set forth more completely in theclaims.

Various advantages, aspects and novel features of the presentdisclosure, as well as details of an illustrated embodiment thereof,will be more fully understood from the following description anddrawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example conventional phaselocked loop (PLL), in connection with an example embodiment of thedisclosure.

FIG. 2 is a block diagram illustrating an examplereference-frequency-insensitive phase locked loop (PLL), in accordancewith an example embodiment of the disclosure.

FIG. 3 is a diagram illustrating an example sampled loop filter (SLPF)for use in a reference-frequency-insensitive phase locked loop (PLL), inaccordance with an example embodiment of the disclosure.

FIG. 4 illustrates timing flows for an example sampled loop filter(SLPF) in a reference-frequency-insensitive phase locked loop (PLL), inaccordance with an example embodiment of the disclosure.

FIG. 5 is a flow chart illustrating example steps for areference-frequency-insensitive phase locked loop, in accordance with anexample embodiment of the disclosure.

FIG. 6 is a flow chart illustrating example steps for implementing areference-frequency-insensitive phase locked loop, in accordance with anexample embodiment of the disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

As utilized herein the terms “circuit” and “circuitry” refer to physicalelectronic components (i.e. hardware) and any software and/or firmware(“code”) which may configure the hardware, be executed by the hardware,and/or otherwise be associated with the hardware. As utilized herein,“and/or” means any one or more of the items in the list joined by“and/or”. As an example, “x and/or y” means any element of thethree-element set {(x), (y), (x, y)}. As another example, “x, y, and/orz” means any element of the seven-element set {(x), (y), (z), (x, y),(x, z), (y, z), (x, y, z)}. As utilized herein, the term “exemplary”means serving as a non-limiting example, instance, or illustration. Asutilized herein, the terms “e.g.,” and “for example” set off lists ofone or more non-limiting examples, instances, or illustrations. Asutilized herein, a device/module/circuitry/etc. is “operable” to performa function whenever the device/module/circuitry/etc. comprises thenecessary hardware and code (if any is necessary) to perform thefunction, regardless of whether performance of the function is disabled,or not enabled, by some user-configurable setting.

Certain embodiments of the disclosure can be found in a method andsystem for a reference-frequency-insensitive phase locked loop. Invarious embodiments of the disclosure, a phase locked loop may beoperable to enable usage of both rising and falling edges of a crystalclock signal generated by a crystal in the phase locked loop for anoperation of the phase locked loop, and perform the operation of thephase locked loop based on the enabling.

In an example embodiment of the disclosure, the phase locked loop may beoperable to generate a reference clock signal whose frequency is twice(or some other multiple of) a frequency of the crystal clock signal andis keyed on both rising and falling edges of the crystal clock signal.The phase locked loop may be operable to enable the usage of both risingand falling edges of the crystal clock signal based on the generatedreference clock signal. In this regard, the phase locked loop may beoperable to generate the reference clock signal utilizing, for example,a frequency doubler in the phase looked loop.

In an example embodiment of the disclosure, the phase locked loop may beoperable to perform a phase comparison function, based on both risingand falling edges of the crystal clock signal, in or during theoperation of the phase locked loop. In this regard, the phase lockedloop may be operable to utilize a sampled loop filter (SLPF) in thephase locked loop during the performing of the phase comparisonfunction. The sampled loop filter (SLPF) may be operable to capturecharge, for example, at a frequency corresponding to the frequency ofthe reference clock signal, from a charge pump (CHP) in the phase lockedloop. The sampled loop filter (SLPF) may be operable to then release thecaptured charge, for example, after each two consecutive highs of thereference clock signal and corresponding to falling edges of the crystalclock signal.

In an example embodiment of the disclosure, the crystal clock signal maycomprise duty cycle errors. The duty cycle errors may result indisturbance at an output of the charge pump (CHP), in or during theoperation of the phase locked loop. In such instances, the phase lockedloop may be operable to eliminate the disturbance associated with theduty cycle errors, utilizing the above mentioned sampled loop filter(SLPF), in or during the operation of the phase locked loop.

FIG. 1 is a block diagram illustrating an example conventional phaselocked loop (PLL) in connection with an example embodiment of thedisclosure. Referring to FIG. 1, there is shown a phase locked loop(PLL) 100. The PLL 100 may comprise suitable logic, circuitry,interfaces and/or code for generating an output signal whose phase maybe related to the phase of an input signal.

In its most basic implementation, a conventional phase locked loop maycomprise, for example, a variable frequency oscillator component and aphase detector, with the frequency oscillator component generating aperiodic signal and the phase detector comparing the phase of thatgenerated signal with the phase of an input signal of the phasedetector—e.g., to adjust the oscillator component generating, based onthe comparison, to keep the phases matched. Bringing an output signalback toward the input signal for comparison is called a feedback loopsince the output is “fed back” toward the input signal forming a loop.Accordingly, PLLs may function based on bringing the output signal back(i.e., ‘feeding back’) toward the input signal for comparison, thusforming a loop. Keeping the input and output phase in lock may alsoallow keeping the input and output frequencies the same. Consequently,in addition to synchronizing signals, a phase locked loop may be used totrack an input frequency, or it can generate a frequency that is amultiple of the input frequency. Accordingly, PLLs may be utilized ascontrol systems or components, providing signals for use in suchoperation as clock synchronization, demodulation, frequency synthesis,and the like. For example, PLLs may be utilized in radio, television,communications, computers and other electronic applications. In thisregard, the PLLs may be utilized in these systems to demodulate signals,recover signals (e.g., from noisy communication channels), generate astable frequency at multiples of an input frequency (e.g., for frequencysynthesis), and/or distribute precisely timed clock pulses (e.g., indigital circuits such as microprocessors).

In the example implementation shown in FIG. 1, the PLL 100 may comprisea crystal (XTAL) 110, a phase frequency detector/charge pump (PFD/CHP)block 120, a loop filter (LPF) 130, a voltage controlled oscillator(VCO) 140, and a divider 150. In this regard, the XTAL 110 may comprisesuitable logic, circuitry, interfaces and/or code that may be operableto generate a periodic crystal clock signal 115 (e.g., based onoscillation in crystal 110).

The PFD (of the PFD/CHP block 120) may comprise suitable logic,circuitry, interfaces and/or code that may be operable to detect thedifference in phase and/or frequency between the crystal clock signal115 (a reference signal) and a feedback signal 155, and generate acorresponding error signal which is proportional to the phase difference(e.g., an error signal for adjusting the frequency at which the VCO 140is operating—i.e., at a higher or lower frequency).

The CHP (of PFD/CHP block 120) may comprise suitable logic, circuitry,interfaces and/or code that may be operable to apply adjustments 125specified by the PFD's error signal—e.g., driving current into LPF 130to ‘up’ (i.e., increase) the frequency, or draw current from the LPF 130to ‘down’ (i.e., lower) the frequency.

The LPF 130 may comprise suitable logic, circuitry, interface and/orcode that may be operable to then apply the changes to the VCO 140, suchas by converting the charges (currents) adjustments 125 applied by theCHP into a control voltage 135 that is used to bias the VCO 140.

The VCO 140 may comprise suitable logic, circuitry, interfaces and/orcode that may be operable to function as an electronic oscillator whoseoscillation frequency is controlled by a voltage input (e.g., thecontrol voltage 135). An output of VCO 140 (in addition to actualintended uses therefore) may be looped back, for use in controllingphase (and frequency) of signals of the PLL 100. In this regard, thedivider 150 may be inserted in the feedback loop to produce a frequencysynthesizer, such as to allow the VCO 140 frequency above the frequencyof the crystal clock signal 115.

In various implementations of the present disclosure, performance ofconventional PLLs may be enhanced, in an optimized manner. This may beachieved by, for example, incorporating additional features/functions,and/or modifying typically utilized ones, to improve performance of thePLL in an efficient manner. For example, in some implementations thefrequency of the crystal signal (i.e., signal outputted by the XTAL 110)may be increased (e.g., doubled) without reducing the frequencyresolution. Furthermore, rather than utilizing a legacy or typical LPF,a sampled loop filter (SLPF) may be utilized, which may enhanceperformance of the PLL because, for example, it may be insensitive toduty cycle errors. Accordingly, incorporation of such features mayresult in improved overall phase noise (PN) associated with alow-frequency crystal (XTAL). A phase noise (PN) may be described, forexample, as a frequency domain representation of rapid, short-term,random fluctuations in a phase of a waveform, caused by time domaininstabilities (e.g., “jitter”).

FIG. 2 is a block diagram illustrating an examplereference-frequency-insensitive phase locked loop (PLL), in accordancewith an example embodiment of the disclosure. Referring to FIG. 2, thereis shown a PLL 200.

The PLL 200 may be substantially similar to the PLL 100 of FIG. 1, forexample, and may comprise the crystal (XTAL) 110, the PFD/CHP block 120,the voltage controlled oscillator (VCO) 140, and the divider 150.However, to enhance performance of the PLL 200 compared to the PLL 100(representing conventional PLL implementation), additional componentsmay be added and/or particular components may be modified. For example,a frequency doubler block 210 may be inserted between the XTAL 110 andthe PFD/CHP 120. Also, the LPF 130 may be replaced with a sampled loopfilter (SLPF) 220.

The frequency doubler 210 may comprise suitable logic, circuitry,interfaces and/or code that may be operable to double the frequency ofthe signals (e.g., the crystal clock signal 115) outputted by the XTAL110, and accordingly, generates a reference clock signal 215. Doing somay enable use of both rising and falling edges of the XTAL 110 clocksignal 115 used for phase comparison (e.g., performed by the PFD of thePFD/CHP block 120).

The SLPF 220 may comprise suitable logic, circuitry, interfaces and/orcode that may be operable to capture charges from the charge pump (CHP)function (of the PFD/CHP 120) at a frequency twice the frequency of thecrystal clock signal 115 (2× Fref). However, the charge is only releasedat the frequency of the crystal clock signal 115 (1× Fref).

The phase frequency detector (PFD) function (of the PFD/CHP 120) wouldremain unchanged. Accordingly, with the effective double Fref (2× Fref),duty cycle error (e.g., none 50% duty cycle) may be absorbed in thefilter capacitors of the SLPF 110, as further described below.

While the implementation described herein is based on doubling the XTALclock, it should be understood that other variations (e.g., M-times theXTAL clock, where M may be an integer) may be implemented in asubstantially similar manner.

FIG. 3 is a diagram illustrating an example sampled loop filter (SLPF)for use in a reference-frequency-insensitive phase locked loop (PLL), inaccordance with an example embodiment of the disclosure. Referring toFIG. 3, there is shown an example sampled loop filter (SLPF) 300. TheSLPF 300 may be substantially similar to or the same as the SLPF 220described with respect to FIG. 2, for example.

In the example implementation shown in FIG. 3, the SLPF 300 maycomprise, for example, three capacitors (C1, C2, and C3) and a singleresistor R1, with a switch (SW 301). In this regard, the switch (SW 301)may be used in managing holding/dumping of charges used in phasecomparisons—e.g., opening the switch (SW 301) holds the charges in C3during the phase comparisons, while closing the switch (SW 301) dumps(releases) the charges to R1, C1, and C2 (a main loop filter 302). TheSLPF 300 may be configured to capture charges from CHP (of the PFD/CHPblock 120) at 2× Fref, as result of the doubling of the crystal clockXTAL 110 (e.g., phase comparisons are done based on both rising andfalling edges 304, 306 of XTAL 110 clock).

The timing flows of the XTAL 110 clock signal 115 and the referenceclock signal 215 (whose frequency is twice the frequency of the XTAL 110clock, and is keyed on both rising and falling edges 304, 306 of theXTAL 110 clock) may correspond to example use scenario. The switch (SW301) of the SLPF 300 may be controlled using a control signal (Sw_ctrl)223. In this regard, when the Sw_ctrl signal 223 is asserted, the switch(SW 301) is asserted, and the charge held in the phase comparison (inthe PFD/CHP block 120) may be dumped or released. Accordingly, Sw_ctrlsignal 223 may be configured, for example, to dump or release the chargeafter each two consecutive/adjacent highs of the signal 215—e.g., may beset up based on the completion of the signal 215 corresponding to thefalling edges 306 of the XTAL 110 clock signal 115, as shown in FIG. 3.

In operation, the phase locked loop (PLL) 200 may be operable to enableusage of both rising and falling edges 304, 306 of the crystal clocksignal 115, which may be generated by the crystal (XTAL) 110 for theoperation of the phase locked loop (PLL) 200. The phase locked loop(PLL) 200 may then be operable to perform the operation(s) based on theabove mentioned enabling.

In an example embodiment of the disclosure, the phase locked loop (PLL)200 may be operable to generate a reference clock signal 215 whosefrequency is twice (or some other multiple of) the frequency of thecrystal clock signal 115 and is keyed on both rising and falling edges304, 306 of the crystal clock signal 115. The phase locked loop (PLL)200 may then be operable to enable the usage of both rising and fallingedges 304, 306 of the crystal clock signal 115 based on the generatedreference clock signal 215. In this regard, the phase locked loop (PLL)200 may be operable to generate the reference clock signal 215utilizing, for example, the frequency doubler 210 in the phase lookedloop (PLL) 200.

During the operation of the phase locked loop (PLL) 200, the phaselocked loop (PLL) 200 may perform a phase comparison function, based onboth rising and falling edges 304, 306 of the crystal clock signal 115.In this regard, the phase locked loop (PLL) 200 may be operable toutilize (in addition to utilizing a phase frequency detector/charge pump(PFD/CHP) 120) a sampled loop filter (SLPF) 220 in the phase locked loop(PLL) 200 during the performing of the phase comparison function. Insuch instances, the sampled loop filter (SLPF) 220 may be operable tocapture charge, for example, at a frequency corresponding to thefrequency of the reference clock signal 215, from the charge pump (CHP)120 in the phase locked loop (PLL) 200. The sampled loop filter (SLPF)220 may then be operable to release the captured charge, for example,after each two consecutive highs of the reference clock signal 215 andcorresponding to falling edges 306 of the crystal clock signal 115.

FIG. 4 illustrates timing flows for an example sampled loop filter(SLPF) in a reference-frequency-insensitive phase locked loop (PLL), inaccordance with an example embodiment of the disclosure.

The timing flows of FIG. 4 show example timing of the XTAL 110 clocksignal 115 and corresponding reference clock signal 215 (2× Fref)resulting from incorporation of the frequency doubler 210 in the PLL 200(along with the Sw_ctrl signal 223 used in controlling the switch (SW301) in the example implementation of the SLPF 300 shown in FIG. 3). Asshown in FIG. 4, duty cycle errors 401 (e.g., none 50% duty cycle) mayresult in a periodic disturbance 402 in locked condition (e.g., as shownin timing flow of V_sample 225, which corresponds to the output of thePFD/CHP 120 and to the input to the SLPF 220). If this disturbance 402is not eliminated, it may result in turn in significant reference spursat an output of the PLL 200. These reference spurs may be considered asunwanted sideband signals. With the sampling performed by a SLPF (e.g.,the SLPF 300), the disturbance 402 may not be seen at the output side ofthe SLPF (e.g., as shown in timing flow of V_tune 230, which correspondsto the output of the SLPF 220).

In operation, the crystal clock signal 115 may comprise, for example,duty cycle errors 401 (e.g., none 50% duty cycle). The duty cycle errors401 may result in disturbance 402 at the output of the charge pump (CHP)120, during the operation of the phase locked loop (PLL) 200. In suchinstances, the phase locked loop (PLL) 200 may eliminate the disturbance402 associated with the duty cycle errors 401, utilizing the abovementioned sampled loop filter (SLPF) 220, during the operation of thephase locked loop (PLL) 200.

FIG. 5 is a flow chart illustrating example steps for areference-frequency-insensitive phase locked loop, in accordance with anexample embodiment of the disclosure. Referring to FIG. 5, the examplesteps start at step 501. In step 502, the phase locked loop (PLL) 200may be operable to enable usage of both rising and falling edges 304,306 of the crystal clock signal 115 generated by the crystal (XTAL) 110in the phase locked loop (PLL) 200 for an operation of the phase lockedloop (PLL) 200. In step 503, the phase locked loop (PLL) 200 may beoperable to perform the operation of the phase locked loop (PLL) 200based on the enabling of using both rising and falling edges 304, 306 ofthe crystal clock signal 115 for the operation. The example steps mayproceed to the end step 504.

FIG. 6 is a flow chart illustrating example steps for implementing areference-frequency-insensitive phase locked loop, in accordance with anexample embodiment of the disclosure. Referring to FIG. 6, the examplesteps start at step 601. In step 602, the phase locked loop (PLL) 200may be operable to generate a reference clock signal 215 whose frequencyis twice (or some other multiple of) a frequency of the crystal clocksignal 115 and may be keyed on both rising and falling edges 304, 306 ofthe crystal clock signal 115. In this regard, the reference clock signal215 may be generated utilizing the frequency doubler 210 in the phaselocked loop (PLL) 200. In step 603, the phase locked loop (PLL) 200 maybe operable to enable the usage of both rising and falling edges 304,306 of the crystal clock signal 115 based on the generated referenceclock signal 215. In step 604, the phase locked loop (PLL) 200 may beoperable to perform a phase comparison function based on both rising andfalling edges 304, 306 of the crystal clock signal 115, in or during theoperation of the phase locked loop (PLL) 200. In this regard, thesampled loop filter (SLPF) 220 may be utilized in the phase locked loop(PLL) 200 during the performing of the phase comparison function.

In step 605, during the performing of the phase comparison function, thesampled loop filter (SLPF) 220 may be operable to capture charge, at afrequency corresponding to the frequency of the reference clock signal215, from the charge pump (CHP) 120 in the phase locked loop (PLL) 200.In step 606, during the performing of the phase comparison function, thesampled loop filter (SLPF) 220 may be operable to release the capturedcharge after each two consecutive highs of the reference clock signal215 and corresponding to falling edges 306 of the crystal clock signal115. The example steps may proceed to the end step 607.

In various embodiments of the disclosure, a phase locked loop such asthe phase locked loop (PLL) 200 may be operable to enable usage of bothrising and falling edges 304, 306 of a crystal clock signal, such as thecrystal clock signal 115, which may be generated by a crystal, such asthe crystal (XTAL) 110, in the phase locked loop (PLL) 200 for anexample operation of the phase locked loop (PLL) 200. The phase lockedloop (PLL) 200 may then be operable to perform the operation of thephase locked loop (PLL) 200 based on the enabling.

The phase locked loop (PLL) 200 may be operable to generate a referenceclock signal 215 whose frequency is twice (or some other multiple of) afrequency of the crystal clock signal 115 and is keyed on both risingand falling edges 304, 306 of the crystal clock signal 115. The phaselocked loop (PLL) 200 may then be operable to enable the usage of bothrising and falling edges 304, 306 of the crystal clock signal 115 basedon the generated reference clock signal 215. In this regard, the phaselocked loop (PLL) 200 may be operable to generate the reference clocksignal 215 utilizing, for example, a frequency doubler such as thefrequency doubler 210 in the phase looked loop (PLL) 200.

The phase locked loop (PLL) 200 may be operable to perform a phasecomparison function, based on both rising and falling edges 304, 306 ofthe crystal clock signal 115, in or during the operation of the phaselocked loop (PLL) 200. In this regard, the phase locked loop (PLL) 200may be operable to utilize (in addition to utilizing a phase frequencydetector/charge pump (PFD/CHP) 120) a sampled loop filter (SLPF) 220 inthe phase locked loop (PLL) 200 during the performing of the phasecomparison function. The sampled loop filter (SLPF) 220 may be operableto capture charge, for example, at a frequency corresponding to thefrequency of the reference clock signal 215, from a charge pump (CHP)120 in the phase locked loop (PLL) 200. The sampled loop filter (SLPF)220 may be operable to then release the captured charge, for example,after each two consecutive highs of the reference clock signal andcorresponding to falling edges 306 of the crystal clock signal 115.

In some instances, the crystal clock signal 115 may comprise, forexample, duty cycle errors 401. The duty cycle errors 401 may result indisturbance 402 at an output of the charge pump (CHP) 120, in or duringthe operation of the phase locked loop (PLL) 200. In such instances, thephase locked loop (PLL) 200 may be operable to eliminate the disturbance402 associated with the duty cycle errors 401, utilizing the abovementioned sampled loop filter (SLPF) 220, in or during the operation ofthe phase locked loop (PLL) 200.

Other embodiments of the disclosure may provide a non-transitorycomputer readable medium and/or storage medium, and/or a non-transitorymachine readable medium and/or storage medium, having stored thereon, amachine code and/or a computer program having at least one code sectionexecutable by a machine and/or a computer, thereby causing the machineand/or computer to perform the steps as described herein for areference-frequency-insensitive phase locked loop.

Accordingly, aspects of the present disclosure may be realized inhardware, software, or a combination of hardware and software. Thepresent disclosure may be realized in a centralized fashion in at leastone computer system or in a distributed fashion where different elementsare spread across several interconnected computer systems. Any kind ofcomputer system or other apparatus adapted for carrying out the methodsdescribed herein is suited. A typical combination of hardware andsoftware may be a general-purpose computer system with a computerprogram that, when being loaded and executed, controls the computersystem such that it carries out the methods described herein.

Aspects of the present disclosure may also be embedded in a computerprogram product, which comprises all the features enabling theimplementation of the methods described herein, and which when loaded ina computer system is able to carry out these methods. Computer programin the present context means any expression, in any language, code ornotation, of a set of instructions intended to cause a system having aninformation processing capability to perform a particular functioneither directly or after either or both of the following: a) conversionto another language, code or notation; b) reproduction in a differentmaterial form.

While the present disclosure has been described with reference tocertain embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the scope of the present disclosure. In addition,many modifications may be made to adapt a particular situation ormaterial to the teachings of the present disclosure without departingfrom its scope. Therefore, it is intended that the present disclosurenot be limited to the particular embodiment disclosed, but that thepresent disclosure will include all embodiments falling within the scopeof the appended claims.

What is claimed is:
 1. A method, comprising: generating, by a phaselocked loop, a reference clock signal whose frequency is twice afrequency of a crystal clock signal and is keyed on both rising andfalling edges of the crystal clock signal, wherein the crystal clocksignal is generated by a crystal in the phase locked loop; andcapturing, by a sampled loop filter (SLPF) in the phase locked loopduring an operation of the phase locked loop, charge from a charge pump(CHP) in the phase locked loop, wherein: the charge is captured at afrequency corresponding to the frequency of the reference clock signal;opening a switch of the sampled loop filter (SLPF) holds the capturedcharge during a phase comparison; and closing the switch releases thecaptured charge.
 2. The method according to claim 1, comprisinggenerating the reference clock signal utilizing a frequency doubler inthe phase locked loop.
 3. The method according to claim 1, wherein thesampled loop filter (SLPF) releases the captured charge after each twoconsecutive highs of the reference clock signal and corresponding tofalling edges of the crystal clock signal.
 4. The method according toclaim 1, comprising controlling the switch utilizing a control signal.5. The method according to claim 4, wherein when the control signal isasserted, the switch is asserted and the captured charge is released. 6.The method according to claim 1, comprising enabling the usage of bothrising and falling edges of the crystal clock signal based on thegenerated reference clock signal.
 7. The method according to claim 6,comprising performing a phase comparison function, based on both risingand falling edges of the crystal clock signal, in the operation of thephase locked loop.
 8. The method according to claim 1, wherein thecrystal clock signal comprises duty cycle errors.
 9. The methodaccording to claim 8, wherein the duty cycle errors from the crystalclock signal result in disturbance at an output of the charge pump(CHP), in the operation of the phase locked loop.
 10. The methodaccording to claim 9, comprising eliminating the disturbance associatedwith the duty cycle errors, utilizing the sampled loop filter (SLPF) inthe operation of the phase locked loop.
 11. A system, comprising: aphase locked loop, the phase locked loop being operable to: generate areference clock signal whose frequency is twice a frequency of a crystalclock signal and is keyed on both rising and falling edges of thecrystal clock signal, wherein the crystal clock signal is generated by acrystal in the phase locked loop; and capture, by a sampled loop filter(SLPF) in the phase locked loop during an operation of the phase lockedloop, charge from a charge pump (CHP) in the phase locked loop, wherein:the charge is captured at a frequency corresponding to the frequency ofthe reference clock signal; opening a switch of the sampled loop filter(SLPF) holds the captured charge during a phase comparison; and closingthe switch releases the captured charge.
 12. The system according toclaim 11, wherein the phase locked loop is operable to generate thereference clock signal utilizing a frequency doubler in the phase lockedloop.
 13. The system according to claim 11, wherein the sampled loopfilter (SLPF) releases the captured charge after each two consecutivehighs of the reference clock signal and corresponding to falling edgesof the crystal clock signal.
 14. The system according to claim 11,wherein the phase locked loop is operable to control the switchutilizing a control signal.
 15. The system according to claim 14,wherein when the control signal is asserted, the switch is asserted andthe captured charge is released.
 16. The system according to claim 11,wherein the phase locked loop is operable to enable the usage of bothrising and falling edges of the crystal clock signal based on thegenerated reference clock signal.
 17. The system according to claim 16,wherein the phase locked loop is operable to perform a phase comparisonfunction, based on both rising and falling edges of the crystal clocksignal, in the operation of the phase locked loop.
 18. The systemaccording to claim 11, wherein the crystal clock signal comprises dutycycle errors.
 19. The system according to claim 18, wherein the dutycycle errors from the crystal clock signal result in disturbance at anoutput of the charge pump (CHP), in the operation of the phase lockedloop.
 20. The system according to claim 19, wherein the phase lockedloop is operable to eliminate the disturbance associated with the dutycycle errors, utilizing the sampled loop filter (SLPF) in the operationof the phase locked loop.